As with the first day of spring, the traditional beginning of autumn starts with an astronomical equinox. This year, the Northern Hemisphere’s autumnal equinox is on Sept. 22 (conversely, the first day of spring in the Southern Hemisphere). Once again, the Earth is perpendicular to the sun, and everyone experiences 12 hours of daylight and 12 hours of night.
In terms of weather, nothing dramatically changes on this date. In fact, since the first day of summer in late June, the North Pole has been tilting farther away from the sun, making the days shorter and the sun’s rays less effective for heating. However, since there’s a lag effect (it takes a while for the Earth’s surface to heat up and then cool down), the warmest temperatures over land tend to occur in July or August. Over water, the warmest temperatures occur as late as September. For convenience, meteorologists consider Sept. 1 to be the start of the three months of autumn. However, typical “autumn weather” is governed by the particular weather patterns that have developed recently and differ every year. Autumn coolness may arrive earlier or later than the calendar date suggests.
Autumn, like spring, is a transition season. Cold fronts from the north make more frequent incursions and reach farther south. For some southern areas, the first cold frontal passage since spring may be the first true sign of autumn. Cool spells often alternate with warm spells, or the weather may just stay cool. Unusually warm, summerlike conditions can occur, especially in early autumn.
As for precipitation, autumn is a transition from summer showers and thunderstorms to the steadier rain associated with fronts and low-pressure areas common in winter. Although snow is often associated with winter, northern and high-altitude areas can experience snow as early as October and, rarely, even in September. While autumn can be dry during this seasonal transition of weather patterns, the actual weather depends on the particular pattern you’re in: warm or cold, wet or dry. On average, it tends to get cooler through autumn.
One of the major changes for plants during autumn is the end of the growing season. This is officially defined as the “first killing freeze.” Annuals die off and perennials prepare to enter a state of dormancy. If the aboveground portion of the plant can’t withstand freezing temperatures, it dies off, leaving the root protected in the soil. Leaves on deciduous trees go through a burst of color, the classic harbinger of autumn, before falling. I discussed this process in a previous article (“Why Do Leaves Change Color in the Fall?” September/October 2019). As with spring frosts, check your local National Weather Service forecast to see when below-freezing temperatures are expected. Your local cooperative extension service will usually have the average date for autumn’s first killing freeze for reference.
Autumn doesn’t mirror spring in terms of severe thunderstorms and tornadoes. While there may be some increase in this type of severe weather, it’s not nearly as active as it is in spring. However, with warm water temperatures lagging behind the cooling air temperatures, hurricanes will continue to form, often becoming the most active in September and continuing to occur as late as November. The National Hurricane Center is the best place for the latest information. Tropical systems can affect more than just the coastal areas. Hurricane Helene traveled hundreds of miles inland in September of 2024, producing catastrophic flooding and wind damage in the southern Appalachians.
Another phenomenon involving hurricanes that can occur in autumn is the transition from a tropical system into an extratropical or hybrid storm. Extratropical storms are similar to winter storms and have a cold air core, unlike warm-core systems, which are characteristic of true tropical systems. However, if a hurricane merges with a strong cold front, the resulting storm can be extremely strong and will weaken slowly as it moves over land.
This was the case in mid-October 1954 when Hurricane Hazel came ashore in North Carolina as a Category 4 storm, became extratropical, and produced record wind speeds and rains while moving into Canada. The damage in Toronto was unprecedented.
Hurricane Sandy, in late October 2012, was a hybrid with characteristics of both tropical and extratropical storms. Its huge size – over 1,000 miles across – brought intense winds from the mid-Atlantic into New England, which resulted in higher-than-normal tides that caused devastating coastal damage.
In the western United States, extending from California northward into western Canada, wildfires are a common weather-related hazard in summer – especially in mountainous terrain – and will continue until weather patterns bring precipitation, which can occur as late as November, or even later in Southern California. In autumn, strong winds become more common, increasing fire risk in non-mountainous areas. Farther east, if there’s a break between summer showers and winter-like weather systems, dry conditions can promote wildfires, especially after leaves have fallen.
Despite these weather hazards, autumn can bring some of the nicest days, offering a break from the heat and humidity of summer and a reprieve from the oncoming, unpleasantly cold weather of winter. Enjoy them!
For more than 30 years, Ed Brotak taught college students about weather, and he’s helped many pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife, Liz.
Originally published in the September/October 2025 issue of Grit magazine and regularly vetted for accuracy.
